Cells contain a cytoskeleton that links intracellular compartments with each other and the plasma membrane. Associations between the cytoskeleton and the lipid membranes bounding these compartments involve spectrin, ankyrin, and integral membrane proteins. Spectrin is a major component of the cytoskeleton and acts as a scaffolding protein. Similarly, ankyrin acts to tether the actin-spectrin moiety to membranes and to regulate the interaction between the cytoskeleton and membranous compartments. Different ankyrin isoforms are specific to different organelles and provide specificity for this interaction. Ankyrin also contains a regulatory domain that can respond to cellular signals, allowing remodeling of the cytoskeleton during the cell cycle and differentiation (Lambert, S. and Bennett, V. (1993) Eur. J. Biochem. 211:1-6).
Ankyrins have three basic structural components. The N-terminal portion of ankyrin consists of a repeated 33-amino acid motif, the ankyrin repeat, which is involved in specific protein-protein interactions. Variable regions within the motif are responsible for specific protein binding, such that different ankyrin repeats are involved in binding to tubulin, anion exchange protein, voltage-gated sodium channel, Na.sup.+ /K.sup.+ -ATPase, and neurofascin. The ankyrin motif is also found in transcription factors, such as NF-.kappa.-B, and in the yeast cell cycle proteins CDC10, SW14, and SW16. Proteins involved in tissue differentiation, such as Drosophila Notch and C. elegans LIN-12 and GLP-1, also contain ankyrin-like repeats. Lux et al. (1990; Nature 344:36-42) suggest that ankyrin-like repeats function as `built-in` ankyrins and form binding sites for integral membrane proteins, tubulin, and other proteins.
The central domain of ankyrin is required for binding spectrin. This domain consists of an acidic region, primarily responsible for binding spectrin, and a basic region. Phosphorylation within the central domain may regulate spectrin binding. The C-terminal domain regulates ankyrin function. The C-terminally-deleted ankyrin, protein 2.2, behaves as a constitutively active ankyrin, displaying increased membrane and spectrin binding. The C-terminal domain is divergent among ankyrin family members, and tissue-specific alternative splicing generates modified C-termini with acidic or basic characteristics (Lambert, supra).
Three ankyrin proteins, ANK1, ANK2, and ANK3, have been described which differ in their tissue-specific and subcellular localization patterns. ANK1, erythrocyte protein 2.1, is involved in protecting red cells from circulatory shear stresses and helping maintain the erythrocyte's unique biconcave shape. An ANK1 deficiency has been linked to hereditary hemolytic anemias, such as hereditary spherocytosis (HS), and a neurodegenerative disorder involving loss of Perkinje cells (Lambert, supra). ANK2 is the major nervous tissue ankyrin. Two alternative splice variants are generated from the ANK2 gene. Brain ankyrin 1 (brank1), which is expressed in adults, is similar to ANK1 in the N-terminal and central domains, but has an entirely dissimilar regulatory domain. An early neuronal form, brank2, includes an additional motif between the spectrin-binding and regulatory domain. An ankyrin homolog in C. elegans, unc-44, produces alternative splice variants similar to ANK2. Mutations in the unc-44 gene affect the direction of axonal outgrowth (Otsuka, A. J. et al. (1995) J. Cell Biol. 129:1081-1092).
ANK3 consists of fourankyrin isoforms (G100, G119, G120, and G195), which localize to intracellular compartments and are implicated in vesicular transport. Ank.sub.G119 is associated with the Golgi, has a truncated N-terminal domain, and lacks a C-terminal regulatory domain. Ank.sub.G120 and Ank.sub.G100 associate with the late endolysosomes in macrophage, lack N-terminal ankyrin repeats, but contain both spectrin-binding and regulatory domains characteristic of ANK1 and ANK2. Ank.sub.G195 is associated with the trans-Golgi network (TGN). These ankyrin isoforms are part of a spectrin complex which may mediate transport of proteins through the Golgi complex. A spectrin-ankyrin-adapter protein trafficking system (SAATS) has been proposed for the selective sequestration of membrane proteins into vesicles destined for transport from the ER to the Golgi and beyond. In this model, intra-Golgi, TGN, and plasma membrane transport would involve exchange of SAATS protein components, including ankyrin isoforms, to specify and distinguish the final destination for vesicular cargo (DeMatteis, M. A. and Morrow, J. S. (1998) Curr. Opin. Cell Biol. 10:542-549).
The discovery of a new human ankyrin family protein and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of autoimmune/inflammatory, cell proliferative, and vesicle trafficking disorders.